The invention relates generally to methods of the formation of persistent joints in superconducting wires, and more specifically, to the formation of persistent joints in magnesium diboride wires.
Typically, magnesium diboride is employed as a superconductor in applications such as magnetic resonance imaging (MRI), generators, motors and fault current limiters. Advantageously, magnesium diboride powders display very strongly linked current flows having large critical current densities (Jc) on the order of 107 A/cm2 in thin films. Additionally, magnesium diboride powders in the shape of wires, tapes or ribbons display Jc values on the order of 105 A/cm2. Further, the upper critical fields (Hc) and irreversibility fields (Hirr) of these powders are greater than about 30 Tesla in thin films.
Typically, magnesium diboride powders are formed by the reaction of elemental magnesium and boron. The result of this process is the production of a fine powder that exhibits high current carrying capabilities at high magnetic fields, properties that are desirable in applications, such as MRI, where large powerful magnets are required. However, the existing methods of making these powders prevent magnesium diboride from achieving the very high operating fields and critical current values, particularly, when processed into wires. This has prevented employing this technology for applications such as MRI. In consequence, these powders should be customized to enable such applications. For example, for MRI applications, it is desirable to have magnesium diboride powders, which may be drawn into thin wires without breaking while employing conventional drawing methods. In some cases, these properties may be achieved by a combination of doping and the addition of other additive materials in the composition of the magnesium diboride powders during processing of the powder. However, this doping and addition process should be carried out in such a way that prevents coating of the particles of the magnesium diboride by non-superconducting impurities. Also, it is desirable to have a uniform dispersion of the additive materials throughout the magnesium diboride powder.
Further, superconductive wires are generally fabricated as units having lengths that are much shorter than the lengths necessary in many applications, such as MRI. Accordingly, superconductive wires may be joined together, end-to-end, to provide a superconductor having sufficient length for a given application. To prevent resistive losses through the length of the wire, the joints connecting two wires should be “persistent.” That is, there should be very low resistance, ideally zero, through the joints. The formation of such persistent joints provides a number of design challenges. The challenges are further amplified with the introduction of magnesium diboride wires.
Accordingly, there is a need for a method of coupling magnesium diboride wires to form wires of sufficient length for applications, such as MRI, where there is minimal resistance at the connection joints.